Increases in train speeds and loading have made railway transportation more efficient. However, this increase also means more arduous duty conditions for the rails, and further improvements in rail material properties are required to make them more tolerant and resistant to the increased stresses and stress cycles imposed. The increase in wear is particularly heavy in tight curves with high traffic density and a greater proportion of freight traffic, and the drop of service life of the rail may become significant and undesirable. However, the service life of the rail has been drastically improved in recent years due to the improvements in heat-treatment technologies for further strengthening the rails, and the development of high strength rails using a eutectoid carbon steel and having a fine pearlitic structure.
In straight and gently curved parts of railroads where lower resistance to wear is required, repeated contacts between wheels and rails may cause rolling contact fatigue (RCF) failures on the surface of the rail head. These failures result from the propagation of fatigue cracks started at the top plane of the rail head surface into the interior thereof. The failures called ‘squat’ or ‘dark spot’ appear mainly, but not exclusively, in the tangent tracks of high-speed railroads and are due to the accumulation of damage on the centre of the rail head surface that results from the repeated contacts between wheels and rails.
These failures can be eliminated by grinding the rail head surface at given intervals. However, the costs of the grinding car and operation are high and the time for grinding is limited by the running schedule of trains.
Another solution is to increase the wear rate of the rail head surface to enable the accumulated damage to wear away before the defects occur. The wear rate of rails can be increased by decreasing their hardness as their wear resistance depends on steel hardness. However, simple reduction of steel hardness causes plastic deformation on the surface of the rail head which, in turn, causes loss of the optimum profile and the occurrence of rolling contact fatigue cracks.
Rails with a bainitic structure wear away more than rails with a pearlitic structure because they consist of finely dispersed carbide particles in a soft ferritic matrix. Wheels running over the rails of bainitic structures, therefore, cause the carbide to readily wear away with the ferritic matrix. The wear thus accelerated removes the fatigue-damaged layer from the rail head surface of the rail head. The low strength of the ferritic matrix can be counter-acted by adding higher percentages of chromium or other alloying elements to provide the required high strength as rolled. However, increased alloy additions are not only costly but may also form a hard and brittle structure in the welded joints between rails. These bainitic steels appear to be more susceptible to stress corrosion cracking and require a more rigid control of residual stresses. Moreover the performance of alumino-thermic and flash butt welding of bainitic steels should be improved.
Rails with a pearlitic structure comprise a combination of soft ferrite and lamellae of hard cementite. On the rail head surface that is in contact with the wheels, soft ferrite is squeezed out to leave only the lamellae of hard cementite. This cementite and the effect of work hardening provide the wear resistance required of rails. The strength of these pearlitic steels is achieved through alloying additions, accelerated cooling or a combination thereof. Using these means, the interlamellar spacing of the pearlite has been reduced. An increase in the hardness of the steel causes an increase in wear resistance. However, at hardness values of about 360 HB and higher, the wear rate is so small that a further increase in hardness does not result in a significantly different wear rate. However, improvements in resistance to rolling contact fatigue have been seen with increasing hardness up to ˜400 HB which is generally regarded as the upper hardness limit for eutectoid and hypo-eutectoid steels with a fully pearlitic microstructure.
However, under practical conditions, the RCF resistance of these high strength pearlitic steels needs to be further improved to delay the initiation of rolling-contact fatigue cracks and thereby prolong the intervals between rail grinding operations.